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Capital expenditures related to oil and natural gas producing activities (Details 2) (USD $) | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Acquisitions: |  |  |  |  |  |
Proved properties | $ 1,817,000 | [1] | $ 839,000 | [1] | $ 3,999,000 | [1]
Unproved properties | 4,608,000 | [1] | 31,109,000 | [1] | 63,354,000 | [1]
Exploration | 26,975,000 | [1] | 235,906,000 | [1] | 41,775,000 | [1]
Development | 355,421,000 | [1] | 275,959,000 | [1] | 161,647,000 | [1]
Total capital expenditures | 388,821,000 | [1] | 543,813,000 | [1] | 270,775,000 | [1]
Net change in property, plant and equipment related to future liabilities for asset retirement obligations that are excluded | (10,700,000) |  | (200,000) |  | (1,800,000) |
Proceeds from Sale of Oil and Gas Property and Equipment | $ 83,600,000 |  | $ 6,000,000 |  | $ 12,400,000 |
[1] | Excludes net additions/(reductions) to property, plant and equipment related to the recognition of future liabilities for asset retirement obligations associated with the plugging and abandonment of oil and natural gas wells, as discussed in Note 10, of $(10.7) million, $(200,000) and $(1.8) million for the years ended December 31, 2013, 2012 and 2011, respectively.

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- Definition Net change in property, plant and equipment related to future liabilitie for asset retirement obligations that are excluded. + References No definition available. + Details
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- Definition Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62038-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 18 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62014-109447 + Details
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- Definition Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62038-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 18 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62014-109447 + Details
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- Definition Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 18 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62014-109447 + Details
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- Definition Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Exploration -URI http://asc.fasb.org/extlink&oid=6512329 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 18 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62014-109447 + Details
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- Definition The cash inflow to dispose of long-lived, physical assets and mineral interests in oil and gas properties used for normal oil and gas operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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